Laser printers
Last updated: May 30, 2007.
Have you ever tried writing with a beam
of light? Sounds
impossible, doesn't it, but it's exactly what a laser printer does
when it makes a permanent copy of data (information) from your
computer on a piece of paper. Thanks to sci-fi and spy movies, we
tend to think of lasers as incredibly
powerful light beams
that can slice through chunks of metal or blast enemy spaceships to
smithereens. But tiny lasers are useful too in a much
more humdrum way: they read sounds and video clips off the discs in
CD and DVD players and they're
vital parts of most office
computers printers. Let's take a closer look at how laser printers
work.
Laser printers and photocopiers
Laser printers are a lot like photocopiers and use the same basic
technology. You might find it helpful to read our article on
how photocopiers work before you read
on.
In a photocopier, a bright
light is used to make an exact copy of a printed page. The light
reflects off the page onto a light-sensitive drum; static electricity
(the effect that makes a balloon stick to your clothes if you rub it a
few times)
makes ink particles stick to the drum; and the ink is then
transferred to paper and “fused” to its surface by hot
rollers. A laser printer works in almost exactly the same way, with
one important difference: because there is no original page to copy,
the laser has to write it out from scratch.
Imagine you're a computer choc full of data. The information you
store is in electronic format: each piece of data is stored
electronically by a microsopically small switching device called a
transistor. The printer's job is
to convert this electronic
data back into words and pictures: in effect, to turn electricity
into ink. With an inkjet printer,
it's easy to see how that
happens: ink guns, operated electrically, fire precise streams of ink
at the page. With a laser printer, things are slightly more complex.
The electronic data from your computer is used to control a laser
beam—and it's the
laser that gets the ink on the page, using static electricity in a
similar way to
a photocopier.
How a laser printer works
When you print something, your computer sends a vast stream of
electronic data (typically a few megabytes or million characters) to
your laser printer. An electronic circuit in the printer figures out
what all this data means and what it needs to look like on the page.
It makes a laser beam scan back and forth across a drum inside the
printer, building up a pattern of static electricity. The static
electricity attracts onto the page a kind of powdered ink called
toner. Finally, as in a photocopier, a fuser unit bonds the toner to
the paper.
- Millions of bytes (characters) of
data stream into the printer from your
computer.
- An electronic circuit in the printer
(effectively, a small computer in its own right) figures out how to
print this data so it looks correct on the page.
- The electronic circuit activates the corona
wire.
This is a high-voltage wire that gives a static electric charge to
anything nearby.
- The corona wire charges up the
photoreceptor drum so the drum gains a
positive charge spread uniformly across its surface.
- At the same time, the circuit activates the laser
to make it draw the image of the page onto the drum.
The laser beam doesn't actually move: it
bounces off a moving mirror that scans it
over the drum.
Where the laser beam hits the drum, it erases the positive charge that
was there and creates an area of negative charge instead.
Gradually, an image of the entire page builds up on the drum: where the
page should be white, there are areas with a positive charge; where the
page should be black, there are areas of negative charge.
- An ink roller touching the
photoreceptor drum coats it with tiny particles of powdered ink
(toner).
The toner has been given an electrical charge, so it sticks to the
parts of the photoreceptor drum that have a negative charge (remember
that opposite electrical charges attract in the same way that opposite
poles of a magnet attract).
No ink is attracted to the parts of the drum that have a positive
charge. An inked image of the page builds up on the drum.
- A sheet of paper from a hopper on the
other side of the printer feeds up toward the drum. As it moves along,
the paper is given a strong electrical charge by another corona wire.
- When the paper moves near the drum, its strong charge attracts
the charged toner particles away from the drum. The image is
transferred from the drum onto the paper but, for the moment, the toner
particles are just resting lightly on the paper's surface.
- The inked paper passes through two hot rollers (the fuser unit). The heat and pressure from the
rollers fuse the toner particles permanently into the fibers of the
paper.
- The printout emerges from the side of
the copier. Thanks to the fuser unit, the paper is still warm. It's
literally hot off the press!
Copyright © Chris Woodford 2007.
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